1. Field of the Invention
This invention relates to improvements in electrophotography and more particularly, to an electrophotographic multiple copying system and to an electrophotographic medium useful therefor.
2. Description of the Prior Art
Electrophotographic processes are well known in the art and described in numerous publications. An early patent describing processes of this nature is U.S. Pat. No. 2,297,691 to C. F. Carlson where a base plate of relatively low electrical resistance such as metal, paper, or the like having a photoconductive insulating surface is electrostatically charged in the dark. The charged coating is then exposed to a light image. The charge leaks off rapidly to the base plate in proportion to the intensity of light to which any given area is exposed. After such exposure, the coating is contacted with electrostatic marking particles in the dark. These articles adhere to the areas where the electrostatic charges remain forming a powder image corresponding to the electrostatic image. The powder image can then be transferred to a sheet of transfer material resulting in a positive or negative print, as the case may be, having excellent detail and quality. Alternatively, where the base plate is relatively inexpensive, as of paper, it may be desirable to fix the powder image directly to the plate itself.
Materials known for the preparation of the photoconductive insulating layers required for the above-described process include selenium, sulphur, zinc oxide, various organic substances such as anthracene or anthraquinone and the like. These materials have been used alone coated onto a substrate or have been dispersed in binders with the resultant dispersions applied to supports, primarily metal foils and paper, to provide an electrophotographic medium.
Attempts have been made to provide electrophotographic media for multiple copying systems using the photoconductive insulating coating materials noted above. However, these photoconductive insulating coating materials are subject to dark decay-- i.e., the conductivity pattern in the light exposed areas decreases rapidly with passage of time and the latent electrostatic image fades until it is ultimately lost. Consequently, for a multiple copying system where an excess of about three copies is desired, it is necessary to re-expose the electrophotographic medium to light in an image pattern prior to each development operation. This involves a decreased speed in the copying process and complex copying equipment.